Human alveolar macrophages have cell surface receptors which are capable of binding at least three human leukocyte granule glycoproteins: Leukocyte elastase, leukocyte chymotrypsin-like enzyme (cathepsin G), and lactoferrin. The three ligands bind to the same surface receptor via a common carbohydrate recognition marker. Planned studies will: 1) Evaluate the potential importance of proteinase binding by mononuclear phagocytes in vitro, utilizing a test system which we have developed to detect solubilization of radio-iodinated plasma fibronectin. (Clin. Res. 29:443A, 1981); 2) evaluate macrophage binding of neutrophil collagenase and myeloperoxidase; and 3) evaluate the fate of internalized proteinase. These studies should provide insight into the importance of macrophages in lung proteinase defenses. The results should also clarify this potentially important type of macrophage-neutrophil interaction.